1. Field of the Invention
The present invention relates to semiconductor integrated circuits and, in particular, to a bipolar transistor structure that includes a low-doped (or undoped) base spacer layer between the emitter and a high-doped base layer and small emitter-base capacitance, thereby resulting in small emitter-base capacitance.
2. Discussion of the Related Art
FIG. 1 shows a conventional silicon-geranium (SiGe) bipolar junction transistor (BJT) 100 that is manufactured using a selectively grown epitaxy (SEG) base. A sketch of the impurity distribution under the emitter polysilicon layer of the FIG. 1 BJT structure is shown in solid lines in FIG. 2.
The FIG. 1 BJT structure includes a collector region 102 that is formed in a shallow trench isolated (STI) portion of a N-type semiconductor (typically silicon) substrate. A P-type epitaxial SiGe base region 104 is formed on the collector region 102. N-doped polysilicon emitter 106 is formed over the base region 104. An epitaxial doped silicon emitter capping layer 108 is formed beneath the emitter 106.
As shown in FIG. 2, the base boron concentration Bhigh of the conventional BJT device is very high, i.e.>5E18/cm3. This results in both a noticeable tunneling current through the emitter-base junction, which contributes to the base current and reduces Beta, and high emitter-base capacitance, which reduces transit frequency fT, especially at low currents.
To reduce emitter-base tunneling in BJT structures, Schuppen et al., “Enhanced SiGi Heterojunction Bipolar Transistors with 160 Gh2-fmax”, IEDM Technical Digest, p. 743 (1995), suggest depositing a low-doped n-type emitter spacer layer on top of the boron-doped base layer, i.e. between the n-type emitter and the p-type base (see FIG. 1). However, molecular beam epitaxy (MBE) is required to implement the necessary dopant profile and MBE is not a commercially-viable manufacturing technique.